Previous histochemical studies from our laboratory have demonstrated that serotonin (5-hydroxytryptamine) in the mammalian thyroid gland is localized in parafollicular cells. We are now studying the uptake of serotonin by the thyroid gland. Bat and mouse thyroid lobes are incubated in Eagles solution which contains tritiated serotonin. The incubating solution is maintained at 37 degrees Centigrade with a mixture of 95 percent O2 and 5 percent CO2 bubbled in. Preliminary data has demonstrated that the uptake of serotonin by the thyroid gland is highly temperature dependent and is inhibited by the addition of sodium fluoride and sodium iodoacetate to the incubating media. However, the addition of inhibitors of aerobic metabolism to the incubating solution does not appear to alter the uptake of tritiated serotonin. Electron microscopy radioautography of incubated thyroid lobes has revealed that parafollicular cells do not take up serotonin. Parafollicular cells however, can take up the serotonin precursor, 5- hydroxytryptophan, and convert it to serotonin. Radioautographs indicated that only the follicular cells take up serotonin itself. Analysis of electron micrographs are in progress to determine if there may be specific association of serotonin with follicular cell organelles.